1. Technical Field of the Invention
The present invention relates to a cationic resin composition capable of providing a cationic electro-deposition coating having excellent weatherability or weather resistance and corrosion resistance, and good long-term bath stability.
2. Description of Related Art
Cationic electro-deposition coating compositions have been used for wide ranging purposes including automobile undercoating and those having various characteristics have been developed.
As inventions intended to satisfy both weather resistance and corrosion resistance, those having an epoxy resin and an acrylic resin and forming, on a steel plate side, a layer rich in the epoxy resin superior in corrosion resistance and, on the film surface side, the other layer rich in the acrylic resin are disclosed in Japanese Laid-Open Patent Publication Nos. 8-333528, 10-292131, 2000-345394 and 2001-234116.
In the cationic electro-deposition coating, a film is formed by filling an electro-deposition bath of 5 to 300 m3 with a cationic electro-deposition coating, applying a voltage to an automobile body or part which is to be coated, thereby depositing a film thereon, and then baking and drying the film.
Application of a cationic electro-deposition coating requires, in addition to an electro-deposition tank, collecting and water washing equipment, UF equipment and precise filtration equipment. The cationic electro-deposition coating receives many stresses such as mechanical one upon circulation or washing of a collected liquid with water, mechanical stress due to a pressure difference upon passing through the filtration equipment, mechanical stress occurring upon suction of a pump in the electro-deposition tank or delivery, and chemical stress owing to contamination by a degreased liquid or chemical liquid brought in by an automobile body or volatilization of a solvent.
When the amount of substances to be coated decreases, a decline in the loss of a coating occurs, leading to a decrease in the amount of the coating to be supplemented. The remaining coating inevitably receives mechanical or chemical stress as described above for a long period of time. For example, 1 turnover/month (replaced ratio: about 65%/month) means that it takes a month to add, to an electro-deposition tank, a coating in an amount to make up for the loss by line coating.
In an electro-deposition line of 0.1 turnover/month (which line may hereinafter be called “low-speed turnover line”), it takes 10 months for 65% of a coating in an electro-deposition tank to be replaced. The remaining 35% of the coating inevitably undergoes a stress for more than 10 months.
For a cationic electro-deposition coating, long-term stability of the coating is therefore important. Application of a conventional cationic electro-deposition coating, which contains resins drastically different in a solubility parameter, to a low-speed turnover coating line sometimes causes problems such as deterioration in coating finish, clogging upon precise filtration, clogging upon ultrafiltration and disturbance of maintenance such as cleaning by agglomerates appearing in an electro-deposition tank. There is accordingly a demand for the development of a cationic electro-deposition coating excellent in long-term stability of the coating and having both weather resistance and corrosion resistance.